Cancer cells must interact with endothelial cells both when they enter and exit the vascular wall. The interaction with endothelial cells may determine whether or not the cancer cells can complete the metastatic process and seed in a distant organ. In this study we used a syngeneic rat model to study in vivo the initial stages of metastatic development which involves tumor cell arrest in the microvasculature and interaction with vascular endothelium. Rat liver epithelial (RLE) cells transformed with raf or raf/myc oncogenes produced spontaneous metastases in syngeneic animals. After introduction of the E. coli lacZ gene, the oncogene transformed rat cells could be visualized as single cells in different organs through histochemical staining for beta galactosidase. Time course experiments following intravenous injections of the raf/myc transfectant demonstrated that a large number of cells arrested in the lungs within minutes after the injection, but relatively few cells developed into microscopic and grossly visible metastases. Spontaneous micro metastases consisting of few cells could be visualized in the lungs six days following subcutaneous injection. For in vitro studies of tumor-endothelial cell interactions, we have established a new approach to isolated pure primary cultures of rat endothelial cells. This involves a magnetic immunobead isolation technique using dynal beads coated with endothelial specific antibodies. So far, pure cultures of sinusoidal liver endothelial cells were isolated using OX-43 monoclonal endothelial antibody coated beads. Studies on cocultures of the rat tumor cells and endothelial cells have revealed modulation of metalloproteinase activity of tumor cells by endothelial